Inductive power transmission has many important applications spanning many industries and markets. FIG. 1 shows a conceptual representation of a resonant inductive power transmission system. In FIG. 1, a source of alternating electrical energy is applied to the primary inductor 100 of an air gap transformer. Magnetic coupling between the transformer primary inductor 100 and the transformer secondary inductor 102 transfers some proportion of the primary side energy to the transformer secondary inductor 102, which is removed by some distance from the primary inductor 100. The primary inductor magnetic field, the primary inductor current, and the secondary inductor current are proportional. Resonance applied to the primary inductor 100 increases primary side inductor current producing a corresponding increase in the magnetic flux, the secondary inductor current and the power transferred from the primary to the secondary.
The magnetic flux from the primary inductor 100 induces a voltage into the winding of secondary inductor 102. Maximum secondary current and therefore maximum power transmission occurs when the secondary inductor winding is resonant as well. The result is a two-pole resonant circuit consisting of two magnetically coupled resonant circuits. The resonant circuits can be parallel resonant with the inductor and capacitor wired in parallel as shown in FIG. 1, or they can be series wired and series resonant. Furthermore, the primary and secondary side resonances need not share the same form.
Efficient resonant inductive wireless power transfer relies upon maintaining a high degree of resonance in both the primary source inductor and a secondary load inductor. However, transformer primary and secondary resonant frequencies are affected by many factors including manufacturing variation, component tolerance, primary-secondary separation distance, axial alignment, temperature and other factors. Efficient resonant inductive wireless power transfer therefore demands continuous, autonomous adjustment in order to maintain the required high degree of resonance.
When providing an inductive (or wireless) source of power to vehicles, for example, these variations are encountered routinely and present a critical problem for manufacturers of electric vehicles and other vehicles that require an external source of power. It is desired to develop a system for charging vehicles that addresses these problems such that the primary inductor winding may be located on or in a horizontal surface and the secondary inductor winding may be attached to the bottom of the vehicle for efficient wireless transfer of electrical power to the vehicle. The present invention addresses these needs in the art.